Extra DNS Server being assigned

Windows 2008 R2 domain, DHCP being handled by PDC. Somehow clients are being assigned an extra DNS Server (that is actually a public DNS server) that is not part of the DHCP scope and is not manually/statically assigned. How could this DNS server setting be pushed out if it's not coming from DHCP?

Who is Participating?

WinXP clients can get their DNS servers from Group Policy (the setting doesn't affect newer OSes). A script (whether run through Group Policy or manually) could also affect this.
Can you provide the results of running ipconfig /all?